(a) Field of the Invention
The present invention relates to a fuel cell system, and more particularly, to a fuel cell system employing a reformer with an improved structure.
(b) Description of the Related Art
A fuel cell is a system for producing electric power. In a fuel cell, chemical reaction energy between oxygen and hydrogen contained in hydrocarbon-group materials such as methanol, ethanol, and natural gas is directly converted into electric energy.
Depending on the type of electrolyte used, the fuel cell is classified into different types, including: phosphate fuel cell, molten carbonate fuel cell, solid oxide fuel cell, and polymer electrolyte or alkali fuel cell. Although each of these different types of fuel cells operates using the same principles, they differ in the type of fuel, catalyst, and electrolyte used, as well as in drive temperature.
A polymer electrolyte membrane fuel cell (PEMFC) has been developed recently. As compared to other fuel cells, the PEMFC has excellent output characteristics, a low operating temperature, and fast starting and response characteristics. The PEMFC has a wide range of applications. It may be used in vehicles, in the home and in buildings, and for the power source in electronic devices.
The basic components of the PEMFC are a stack, reformer, fuel tank, and fuel pump. The stack forms the main body of the fuel cell. The fuel pump supplies fuel in the fuel tank to the reformer. The reformer reforms the fuel to generate hydrogen gas, and supplies the hydrogen gas to the stack. Accordingly, the PEMFC sends the fuel in the fuel tank to the reformer by operation of the fuel pump, the fuel is reformed in the reformer to generate hydrogen gas, and the hydrogen gas undergoes an electrochemical reaction with oxygen in the stack to thereby generate electric energy.
In the above fuel cell system, the stack (where the generation of electricity takes place) is structured to include a few to a few tens of unit cells realized with a membrane electrode assembly (MEA), with separators provided on both sides thereof. In the MEA, an anode electrode and a cathode electrode are provided opposing one another with an electrolyte layer interposed therebetween. Further, the separator is typically realized using what is referred to in the art as a bipolar plate, and acts to separate each of the MEAs. The separator also functions to provide a pathway through which hydrogen gas and oxygen, which are required for fuel cell reaction, are supplied to the anode electrode and cathode electrode of the MEA. In addition, the separator functions as a conductor for connecting the anode electrode and cathode electrode of each MEA in series. Accordingly, hydrogen gas is supplied to the anode electrode and oxygen is supplied to the cathode electrode via the separator. Through this process, an oxidation reaction of the hydrogen gas occurs in the anode electrode, and a reduction reaction of the oxygen occurs in the cathode electrode. Electricity is generated by the movement of electrons occurring during this process. Heat and moisture are also generated.
The reformer in the fuel cell system described above is a device that generates hydrogen gas from fuel containing the hydrogen through a chemical catalytic reaction realized by heat energy. The reformer typically includes a reforming reactor that generates the heat energy and hydrogen gas from fuel, and a reducer for reducing the concentration of carbon monoxide contained in the hydrogen gas. The reforming reactor utilizes heat generation and heat absorption characteristics employing catalytic processes. In particular, the reforming reactor includes a heat generator for creating reaction heat through a catalytic oxidation reaction, and a heat absorber that receives the reaction heat and generates hydrogen gas through a catalytic reforming reaction.
However, since in the conventional reformer the heat generator and heat absorber are provided as independent units, efficiency of heat transfer is reduced as a result of heat exchange not occurring directly between the heat generator and heat absorber. This separate formation of the heat generator and heat absorber also increases the overall size of the system. Finally, fuel supplied to the reformer is pre-heated during the initial operation of the conventional fuel cell system, and the energy required for this process acts to reduce the overall efficiency of the system.